This invention relates, in general, to the manufacture of semiconductor devices, and more particularly, to a method of reducing the number of impurities in semiconductor wafers.
Impurities are introduced into semiconductor wafers during the fabrication of the wafers and the fabrication of semiconductor devices on the wafers. The removal of impurities from semiconductor wafers is important in order to prevent the formation of defects, such as oxidation induced stacking faults. Transition metal impurities also cause unfavorable electrical characteristics in the devices, such as high junction leakage.
Several methods have been used to keep impurities away from the active device region on semiconductor wafers. The ability of defects to capture impurities is called gettering. One such gettering technique involves intentionally damaging the back surface of the wafer. Mechanical abrasion, such as lapping or sand blasting, have been used for this purpose. The damage may also be created by a laser beam or ion implantation. The damage creates dislocations in the silicon lattice which represent favorable trapping sites for impurities, such as metallics, and other bulk defects, such as stacking faults.
Another method, called intrinsic gettering, uses oxygen precipitation for trapping sites. Wafers having oxygen concentrations close to the solid solubility are subjected to high temperature which removes oxygen from the surface of the wafer by out-diffusion. The wafers are then annealed at a lower temperature and SiO.sub.x precipitates form in the interior where a saturated amount of oxygen is present. The SiO.sub.x precipitates act as a sink for impurities and defects, thus a denuded or defect-free zone is created near the surface where the semiconductor device is to be fabricated.
Other techniques for gettering include the use of a silicon dioxide or silicon nitride layer grown on the back surface of the semiconductor wafer. These layers are not single crystalline layers, therefore, the polycrystalline grain boundaries act as the trapping sites for impurities. The silicon dioxide or silicon nitride layer is typically lapped off near the end of the processing cycle.
The gettering techniques described above are used during the device processing, therefore, care must be taken not to anneal the damage out during the device processing. If the damage is annealed, the impurities will no longer be trapped and can diffuse throughout the semiconductor wafer. To avoid this, gettering treatments are sometimes applied towards the end of the process cycle. However, at this point the crystalline lattice may already be damaged, and impurities may be located at defects where they are difficult to getter away from. If these impurities were removed before semiconductor processing began, only those impurities introduced during the device manufacture would have to be gettered. In addition, if an epitaxial silicon layer is grown on the wafer, defects from the wafer will propagate to the epitaxial layer. If a defect/impurity free wafer is provided, higher quality epitaxial silicon will be grown, which provides for higher quality semiconductor devices fabricated thereon.
By now it should be appreciated that it would be advantageous to provide a process that not only getters impurities, but removes impurities from the semiconductor wafer.
Accordingly, it is an object of the present invention to provide a process which removes impurities and defects from semiconductor wafers.
Another object of the present invention is to provide a process which provides for improved quality of semiconductor wafers and resultant semiconductor devices.
A further object of the present invention is to provide a gettering process which is easily incorporated into the process flow of fabricating semiconductor wafers.